Gypsum is also known as calcium sulfate dihydrate, terra alba or landplaster. Calcined gypsum is obtained by removing part of the water associated with the gypsum crystal. Synonymous terms for calcined gypsum are Plaster of Paris, stucco, calcium sulfate half-hydrate and calcium sulfate hemihydrate. Calcined gypsum, stucco and hemihydrate are the most commonly used terms, and they are used interchangeably in this application. When gypsum is mined, the natural rock is found in the dihydrate form, having about two water molecules associated with each molecule of calcium sulfate. In order to produce the stucco form, the gypsum can be calcined to drive off some of the water of hydration represented by the following equation:CaSO4.2H2O→CaSO4.1/2H2O+3/2H2O
Calcium sulfate hemihydrate is obtained by calcination to remove the associated water molecules. The hemihydrate is produced in at least two crystal forms. Alpha-calcined gypsum is made by a slurry process or a lump rock process whereby the calcium sulfate dihydrate is calcined under pressure. The alpha-calcined gypsum forms less acicular crystals than beta-calcined gypsum, allowing the crystals to pack tightly together, making a denser and stronger plaster. The crystal morphology of alpha hemihydrate allows water to flow easily between the crystals, requiring less water to form a flowable slurry. More elongated irregular shaped crystals are characteristic of the beta-hemihydrate, which is obtained by calcining gypsum at atmospheric pressure. This crystal structure results in a less dense product because the crystals are more loosely packed. The beta form also requires more water to fluidize the calcined gypsum. If the calcining of the dihydrate is performed at ambient pressure, the beta form is obtained and the cost is relatively low compared to the alpha-calcined gypsum.
Synthetic gypsum, which is a byproduct of flue gas desulfurization processes from power plants, is also useful. Flue gas that includes sulfur dioxide is wet scrubbed with lime or limestone. Calcium from the lime combines with the sulfur dioxide to form calcium sulfite.CaCO3+SO2→CaSO3+CO2 Via forced oxidation, the calcium sulfite is converted to calcium sulfate.
            CaSO      3        +          2      ⁢              H        2            ⁢      O        +                  1        2            ⁢              O        2              →                    CaSO        4            ·      2        ⁢          H      2        ⁢    O  Synthetic gypsum is converted to a hemihydrate by calcination, described above.
A number of useful gypsum products can be made by mixing the calcium sulfate hemihydrate with water and shaping the resulting product slurry into the desired shape. The product slurry is permitted to set by allowing the calcium sulfate hemihydrate to react with sufficient water to convert the hemihydrate into a matrix of interlocking dihydrate crystals. As the matrix forms, the product slurry becomes firm and holds the desired shape. Excess water must then be removed from the product by drying.
Set accelerators and set retarders (collectively known as “set modifiers”) are used in gypsum product compositions to allow control of the set time. If the set time is too long, contractors spend time waiting for the composition to set before they can move on to the next step of the project. When gypsum sets too quickly, the composition hardens before it is properly finished. In such cases, the surface may not be as smooth as desired or the product may not have been “worked” enough to produce a good finish.
Dry mixtures of calcium sulfate hemihydrate with additives are pre-blended and produced for the convenience of contractors and others who may not have knowledge of the need for additives or the amounts of additives that would be useful. The dry mixtures are designed to be mixed with water to produce a high quality composition that is easy to use. One example of a setting mixture is one that produces a flooring slurry when combined with water, such as LEVELROCK® gypsum flooring material.
Set accelerators are utilized to hasten setting of the slurry. Retarders are added to setting mixtures to increase the working time of the slurry. The working time, also known as the open time, is the time that the slurry is pliable and can be shaped into a desired configuration. In the flooring slurry, a target working time is one that is sufficiently long to allow the technician to adequately level the floor. Without a set retarder, a calcium sulfate hemihydrate (stucco) slurry has a working time that is often insufficient for professional finishers to produce a satisfactory floor. The set retarders extend the working time, depending on the composition being used, and where and how the slurry is being applied, so that the finisher has the time to work the slurry to make a high quality floor.
Conventionally, proteinaceous retarders, such as SUMA retarder, and non-proteinaceous retarders, such as cream of tartar (potassium bitartrate), sodium citrate and diethylene-triamine-pentacetic acid, are utilized in flooring and other gypsum-based setting compositions to provide set control. Suma retarder, commercially available as a dry powder additive, has disadvantages associated with its use, but no suitable replacement has been available. For example, SUMA has a strong and offensive odor. It is proteinaceous, and sourced from hair and hooves of various animals, such as horses. SUMA also has undesirable ageing characteristics. There is a long-standing need in the industry for a suitable replacement for this dry set retarder.
DTPAs are also well known as good set retarders, but the dry powdered forms have been found to be ineffective. For example, U.S. Pat. No. 4,661,161 to Jacacki (“Jacacki”) teaches the addition of a liquid form of diethylenetriamine pentaacetic acid (“DTPA”). There is a need for a dry or powder component useful in setting mixtures that utilizes a DTPA as the set retarder that could be a suitable replacement for proteinaceous set retarders, such as SUMA. There is also a related need to improve the efficacy of liquid DTPA as a suitable set retarder.
There is also a need for a setting mixture that has good compressive strength. For example, gypsum products must be able to grip fasteners that penetrate the product or hold up to the pressures to which a floor is subjected.
Further there is a need for a setting mixture that requires less water to form a flowable slurry. Water is not always conveniently available at a job site. Use of reduced amounts of water also reduces the drying time of the product. Where the product is dried in an oven or kiln, the amount of fuel needed to dry the product can be reduced, realizing a savings in fuel cost.